Boundary scan

Boundary scan is a method for testing interconnects (wire lines) on printed circuit boards or sub-blocks inside an integrated circuit. Boundary scan is also widely used as a debugging method to watch integrated circuit pin states, measure voltage, or analyze sub-blocks inside an integrated circuit.

The Joint Test Action Group (JTAG) developed a specification for boundary scan testing that was standardized in 1990 as the IEEE Std. 1149.1-1990. In 1994, a supplement that contains a description of the Boundary Scan Description Language (BSDL) was added which describes the boundary-scan logic content of IEEE Std 1149.1 compliant devices. Since then, this standard has been adopted by electronic device companies all over the world. Boundary scan is now mostly synonymous with JTAG.

The boundary scan architecture provides a means to test interconnects (including clusters of logic, memories, etc.) without using physical test probes; this involves the addition of at least one test cell that is connected to each pin of the device and that can selectively override the functionality of that pin. Each test cell may be programmed via the JTAG scan chain to drive a signal onto a pin and thus across an individual trace on the board; the cell at the destination of the board trace can then be read, verifying that the board trace properly connects the two pins. If the trace is shorted to another signal or if the trace is open, the correct signal value does not show up at the destination pin, indicating a fault.

To provide the boundary scan capability, IC vendors add additional logic to each of their devices, including scan cells for each of the external traces. These cells are then connected together to form the external boundary scan shift register (BSR), and combined with JTAG TAP (Test Access Port) controller support comprising four (or sometimes more) additional pins plus control circuitry.

Some TAP controllers support scan chains between on-chip logical design blocks, with JTAG instructions which operate on those internal scan chains instead of the BSR. This can allow those integrated components to be tested as if they were separate chips on a board. On-chip debugging solutions are heavy users of such internal scan chains.

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